Photodiode and method for fabricating same

ABSTRACT

A Schottky photodiode includes a semiconductor layer and a conductive film provided in contact with the semiconductor layer. The conductive film has an aperture and a periodic structure provided around said aperture for producing a resonant state by an excited surface plasmon in a film surface of the conductive film by means of the incident light to the film surface. The photodiode detects near-field light that is generated by at the interface between the conductive film and semiconductor layer the excited surface plasmon. The aperture has a diameter smaller than the wavelength of the incident light.

TECHNICAL FIELD

The present invention relates to a photodiode for converting lightsignals that include invisible light such as infrared light toelectrical signals at high speed, and to a method for fabricating thesame.

BACKGROUND ART

Photodiodes are frequently used as devices for converting opticalsignals to electrical signals at high speed. Photodiodes areindispensable in the fields of information processing and communication.

Various types of photodiodes are known, but a representative example ofa photodiode for operating at high speed is the pin-type photodiode. Asshown in FIG. 1, a pin-type photodiode is constituted from asemiconductor such as silicon and is of a configuration in which i-layer(intrinsic semiconductor layer) 51 is interposed between p-layer (p-typesemiconductor layer) 52 and n-layer (n-type semiconductor layer) 53.P-layer 52 is formed as a thin layer on a portion of the surface ofi-layer 51, and first electrode (anode electrode) 54 with window 59formed in its center is provided so as to both enclose the periphery ofp-layer 52 and contact p-layer 52. Second electrode (cathode electrode)55 is provided on the surface of n-layer 53 that is not the i-layer 51side. P-layer 52 is exposed on the bottom surface of window 59, andantireflection film 58 is provided on the exposed surface of p-layer 52.

Load resistance 50 and this pin-type photodiode are connected in aseries to bias power supply 56, and when a reverse bias voltage isapplied to the photodiode by bias power supply 56 such that thefirst-electrode 54 side is negative and the second-electrode 55 side ispositive, substantially the entire region of i-layer 51 of highresistance becomes a depletion layer of the electric charge carrier.When incident light 57 irradiates the interior of the photodiode by wayof window 59 in this state, the photons of incident light 57 are mainlyabsorbed by i-layer 51 to generate electron-hole pairs. The generatedelectrons and holes each drift in opposite directions within thedepletion layer under the influence of the reverse bias voltage togenerate a current, and are detected as a signal voltage between bothends of load resistance 50.

The chief factors that limit the response speed of the photoelectricconversion of such a pin-type photodiode are a circuit time constantthat is determined by the product of load resistance 50 and theelectrical capacitance produced by the depletion layer and the carriertransit time needed for the electrons and holes to pass through thedepletion layer. Thus, in order to improve the response time, either thecircuit time constant should be reduced or the carrier transit timeshould be shortened.

Schottky photodiodes are sometimes used to improve the response speed byshortening the carrier transit time. As shown in FIG. 2, a Schottkyphotodiode is composed of a semiconductor such as silicon, n⁻-typesemiconductor layer 61 being formed on n⁺-type semiconductor layer 60,and further, semi-transparent metal film 66 being provided on a portionof the surface of n⁻-type semiconductor layer 61 so as to contactn⁻-type semiconductor layer 61. Semi-transparent metal film 66 is ametal thin-film thin enough to transmit incident light 67. Firstelectrode 62 in which window 69 is formed in the center is provided soas to both surround the periphery of semi-transparent metal film 66 andcontact semi-transparent metal film 66. Second electrode 63 is providedon the surface of n⁺-type semiconductor layer 60 that is not then⁻-type-semiconductor layer 61 side. Semi-transparent metal film 66 isexposed on the bottom of window 69, and antireflection film 68 isprovided on the exposed surface of semi-transparent metal film 66. As inthe case of the pin-type photodiode shown in FIG. 1, a reverse biasvoltage is applied by way of bias power supply 64 and load resistance 65to first electrode 62 and second electrode 63.

In this type of Schottky photodiode, a Schottky barrier is generated inthe vicinity of the interface in which n⁻-type semiconductor layer 61contacts semi-transparent metal film 66. In the vicinity of thisSchottky barrier, electrons are diffused from semi-transparent metalfilm 66 and toward n⁻-type semiconductor layer 61 to generate adepletion layer. When incident light 67 is irradiated in this state,electrons are generated in n⁻-type semiconductor layer 61, and theseelectrons drift within the depletion layer under the influence of thereverse bias voltage. The drift of the electrons within the depletionlayer generates a current and is detected as a signal voltage betweenthe two ends of the load resistance 65.

In contrast to a pin-type photodiode such as shown in FIG. 1 in whichi-layer 51 for photon absorption, i.e., the depletion layer, must beprovided with sufficient thickness, the depletion layer in a Schottkyphotodiode can be made thin, and as a result, the carrier transit timecan be shortened. In addition, the light absorption in the devicesurface layer can be effectively used in a Schottky photodiode.

On the other hand, when the circuit time constant is reduced to improvethe response speed of photoelectric conversion, either the loadresistance should be reduced or the electrical capacitance of thedepletion layer should be reduced. However, when the load resistance isreduced to shorten the circuit time constant, the voltage level of theregeneration signal that can be extracted drops, the device becomes moresusceptible to the effect of thermal noise and other noises, and the SNratio (signal-to-noise ratio) is degraded. Thus, reducing the electricalcapacitance of the depletion layer results in the necessity to improvethe SN ratio of the regeneration signal to reduce read errors. Inparticular, when the depletion layer is thinned to shorten the carriertransit time, the electrical capacitance increases, and as a result, thearea of the depletion layer or the Schottky junction must be reduced toobtain higher speeds. However, decreasing the junction area reduces theutilization of the signal light, and this in turn gives rise to theproblem of degradation of the SN ratio of the regeneration signal.Although a lens can be used to condense and improve the utilization ofthe signal light, the provision of a lens not only increases the size ofthe photoelectric conversion device itself, but also entails thedifficult operations of aligning the lens and photodiode and positioningthe lens and optical fiber.

In response to these problems, various attempts have been made with thedevelopment of technology in recent years to achieve a photoelectricconversion device of this type that is capable of higher speeds and morecompact sizes than the prior art through the use of a metal surfaceplasmon

JP-A-59-108376 (Patent Document 1) discloses a photodetector that iscomposed of a metal/semiconductor/metal (MSM) device in which twoelectrodes are disposed on the same surface of a semiconductor. This MSMphotodetector is generally a type of Schottky photodiode having aSchottky barrier in the vicinity of the two electrodes. A portion of thelight transmitted by the electrodes is absorbed; by the semiconductor togenerate free electrons. This type of MSM photodetector suffers from theproblem that increasing the thickness of the semiconductor to raise thequantum efficiency causes an increase in the propagation distance ofelectrons and a consequent drop in the operation speed. To prevent thisdrop in operation speed, JP-A-59-108376 discloses an arrangement inwhich metal electrodes are provided along periodic surfaceirregularities to achieve efficient coupling of incident light and thesurface plasmon of the metal electrodes and propagate the light withinthe photodetector. As one method of application to the fabrication ofthe above-described MSM photoreception device, JP-A-08-204225 (PatentDocument 2) discloses a method for forming a metal film on asemiconductor and then oxidizing a portion of the :metal film to form aphototransmissive insulation pattern.

A photoreception device that detects near-field light has also beenproposed. JP-A-08-204226 (Patent Document 3) discloses an MSMphotoreception device having a pair of conductive voltage applicationmembers on the same surface of a semiconductor in which the width of aphototransmissive insulation pattern separating the pair of conductivevoltage application members is set to a dimension equal to or less thanthe wavelength and in which the near-field light generated from the endsof conductive voltage application members on the both sides of thephototransmissive insulation pattern is used to raise the response speedof photodetection. The conductive voltage application members aregenerally constituted by a metal film. In this configuration, the widthof the opening for generating near-field light determines the efficiencyand the distance of drift of electrons in the depletion layer determinesthe response speed, but because the width of the phototransmissiveinsulation pattern is the width of the depletion layer as a Schottkyphotodiode, the width of the opening and the distance of drift ofelectrons cannot be independently set and high efficiency and high speedtherefore cannot be simultaneously obtained in the photoreceptiondevice.

JP-A-10-509806 (Patent Document 4) discloses a photoelectric coupler inwhich the surface plasmon phenomenon is used. In this photoelectriccoupler, a device configuration is employed in which interdigital metalelectrodes aligned with regular spacing on a semiconductor are arrangedsuch that positive electrodes and negative electrodes confront eachother with one fitting into the other. By means of this deviceconfiguration, incident light, transmitted light, reflected Flight,surface plasmon and the like are coupled with each other by resonance.In an MSM photoreception device of this type that uses photoelectriccoupling technology, free electrons generated by incident light areenhanced by the coupling of incident light and surface plasmon, but whenthe area irradiated by incident light is reduced to reduce the electriccapacitance of the depletion layer, the strength of the detected signalfalls and the SN ratio drops.

JP-A-2002-076410 (Patent Document 5) discloses a photovoltaic device forconverting the energy of sunlight to electrical energy in which aplurality of micro-semiconductors having a spherical or hemisphericalshape and having pn bonding are used, each semiconductor sphere beinginterposed between a pair of electrodes and periodically arrangedopenings or depressions being provided on one of the pair of electrodes.The periodic shape provided on one of the electrodes causes the incidentlight and the surface plasmon to resonate, thus improving thephotoelectric conversion efficiency of photovoltaic device as a whole.However, this technique relates to a photovoltaic device, i.e., a solarbattery, in which high speed is not required in the response speed ofphotoelectric conversion. As a result, no investigation has beenconducted into reducing the thickness of the depletion layer or reducingthe size of the photoelectric conversion area to achieve an increase inthe speed of photoelectric conversion.

As a device that uses the interaction of incident light and a surfaceplasmon, JP-A-2000-171763 (Patent Document 6) discloses an opticaltransmission device in which a metal film having an aperture andperiodic surface variations is used to greatly enhance the intensity oflight that is propagated through the aperture. This publication statesthat, even with a single aperture, the provision of rows of periodicgrooves around the aperture enables greater enhancement of light that ispropagated through the aperture than a case that lacks periodic rows ofgrooves. However, it is known that in surface plasmon resonance, thetotal energy of transmitted light is attenuated compared to the incidentlight energy. According to Tineke Thio, H. J. Lezec, T. W. Ebbesen, K.M. Pellerin, G. D. Lewen, A. Nahata, and R. A. Linke in “Giant opticaltransmission of sub-wavelength apertures: physics and applications,”Nanotechnology, Vol. 13, pp. 429-432 (Non-Patent Document 1), the totalenergy of light that is transmitted through an aperture having adiameter of 40% of the wavelength or less is attenuated to 1% or less ofthe incident light energy despite the use of surface plasmon resonance.As a result, a high SN ratio cannot be obtained in photoelectricconversion even when the optical transmission device disclosed inJP-A-2000-171763 is used to irradiate light that has been propagatedfrom the aperture of the optical transmission device onto aphotoreception device.

JP-A-2001-291265 (Patent Document 7) discloses a read/write head for anoptical data recording medium that, by using near-field optics toimprove the recording density of an optical data storage medium, bothirradiates light onto an optical recording medium by way of an openinghaving diameter equal to or less than the wavelength and enhances theintensity of light transmitted through the aperture by means of theabove-described surface plasmon resonance. In JP-A-2004-061880 (PatentDocument 8), it is described that the read/write head disclosed in theabove-described JP-A-2001-291265 does not use transmitted light that haspassed through the aperture and then propagated to a remote location,but rather, uses a minute light spot that is formed in proximity to theaperture by near-field light (evanescent light). In the case of anoptical data storage medium, the absorption coefficient of light in thestorage medium can be raised to a high level, whereby all photons withina minute range such as a light spot produced by near-field light can beabsorbed in the storage medium to enable the formation of minuterecording pits. However, it is believed that when near-field light isintroduced into a photodiode, the relatively low light absorptioncoefficient of the material that makes up the photodiode preventsirradiation of light to positions deep in the photodiode, whereby asufficient photodetection current is not observed.

The reference documents cited in the present specification are listedbelow:

Patent Document 1: Japanese Patent Application Laid-open No.Sho-59-108376 (JP-A-59-108376);

Patent Document 2: Japanese Patent Application Laid-open No.Hei-8-204225 (JP-A-08-204225);

Patent Document 3: Japanese Patent Application Laid-open No.Hei-8-204226 (JP-A-08-204226);

Patent Document 4: Japanese Patent Application Laid-open No.Hei-10-509806 (JP-A-10-509806);

Patent Document 5: Japanese Patent Application Laid-open No. 2002-76410(JP-A-2002-076410);

Patent Document 6: Japanese Patent Application Laid-open No. 2000-171763(JP-A-2000-171763);

Patent Document 7: Japanese Patent Application Laid-open No. 2001-291265(JP-A-2001-291265);

Patent Document 8: Japanese Patent Application Laid-open No. 2004-61880(JP-A-2004-061880);

Non-Patent Document 1: Tineke Thio, H. J. Lezec, T. W. Ebbesen, K. M.Pellerin, G. D. Lewen, A. Nahata, and R. A. Linke, “Giant opticaltransmission of sub-wavelength apertures: physics and applications,”Nanotechnology, Vol. 13, pp. 429-432.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Increasing the speed of response of a photodiode necessitates reducingthe thickness of the depletion layer to shorten the carrier transit timeand reducing the area of the depletion layer to decrease the circuittime constant. However, the adoption of these measures results in a dropin the quantum efficiency of converting the photons of incident light toelectron-hole pairs, i.e., the utilization of the signal light, andentails the problem of a drop in the SN ratio. In particular, when thesize of the window of light incidence is set to a size equal to or lessthan the wavelength to reduce the area of the depletion layer, theintensity of light that is transmitted is greatly attenuated due to thediffraction limit, and it is therefore nearly impossible to set thejunction area of the photodiode to 1 μm² or less. Even when an apertureis provided in a metal film, a periodic structure is provided in themetal film in the vicinity of the aperture, surface plasmon resonance isused to raise the intensity of light transmitted through the aperture,and the light that has been thus intensified then introduced into thephotodiode, a drop in the SN ratio is observed and sufficient lightintensity is not obtained. Even when attempting to detect near-fieldlight that is formed in the vicinity of an aperture provided in a metalfilm, it is believed that the relatively low light absorptioncoefficient of the semiconductor material that makes up the photodiodeprevents sufficient light intensity from being obtained.

In this type of photoreception device, the dark current that flows evenin the absence of a signal light becomes a problematic source of noise.Dark current flows as a result of the generation of charge carrier dueto heat and the like and is therefore highly dependent on temperature.The greater the volume of the region that generates electron-hole pairs,the greater the dark current. The inability to implement latticematching of the metal and semiconductor in a Schottky photodiode furtherproduces a type of lattice defect in the depletion layer. This defectacts as the generation center of charge carrier and thus acts toincrease dark current.

It is an object of the present invention to provide a device structurefor increasing the response speed of a photodiode while maintainingsignal intensity.

It is another object of the present invention to provide a photodiodethat, compared to the prior art, not only provides a structure thatfacilitates integration by greatly miniaturizing the device, but thatfurther enables higher speeds, lower power consumption, and lower noise.

Means For Solving The Problem

According to the first aspect of the present invention, a photodiodeincludes: a conductive film having an aperture having a diameter smallerthan the wavelength of incident light and a periodic structure providedaround the aperture for producing a resonant state by an excited surfaceplasmon in a film surface of the conductive film by means of theincident light to the film surface; and a semiconductor layer providedin the vicinity of the aperture of the conductive film and in contactwith the conductive film; wherein the photodiode detects near-fieldlight that is generated at an interface between the conductive film andthe semiconductor layer by the excited surface. In this photodiode, thediameter of the aperture is preferably at least 1/10 the wavelength ofincident light but not greater than ½ the wavelength of incident light.The region in which a Schottky barrier formed by the conductive film andthe semiconductor layer appears preferably substantially matches aregion of generation of the near-field light. The periodic structure ispreferably composed of surface irregularities that have a period in thedirection of increasing distance from the aperture.

According to the second mode of the present invention, a photodiode ischaracterized in that it comprises: a conductive film having a firstsurface and a second surface and including an aperture having a diametersmaller than the wavelength of incident light that is formed from thefirst surface side; and a periodic structure composed of surfaceirregularities having a period in the direction of increasing distancefrom the aperture; a first semiconductor layer of one conductive typeprovided in the vicinity of the aperture of the conductive film and incontact with the second surface of the conductive film; and a secondsemiconductor layer of the conductive type in which concentration ofimpurities is higher than in the first semiconductor layer, and whichcontacts a surface of the first semiconductor layer that is opposite toanother surface in contact with the second surface of the conductivefilm.

The photodiode may further include a first electrode electricallyconnected to the first semiconductor layer and a second electrodeelectrically connected to the conductive film for applying a reversebias voltage to form a Schottky barrier in the vicinity of the junctionwith the conductive film of the second semiconductor layer. In addition,the thickness of the second semiconductor layer interposed between thefirst semiconductor layer and the conductive film is preferably equal toor less than the length of the bleeding of near-field light that appearson the first-surface side at the location of the aperture when light isirradiated from the second surface onto the conductive film. Morespecifically, the thickness of the second semiconductor layer is, forexample, equal to or greater than 50 nm and equal to or less than 100nm.

In this photodiode, the conductive film is preferably composed of ametal film, and the surface irregularities are preferably formed on thefirst surface. The periodic structure is composed of, for example,concentric grooves that take the aperture as center. The aperturepreferably has a diameter of at least 1/10 but no greater than ½ thewavelength of incident light.

In the photodiode of the present invention, a conductive member may beincluded having a dimension that is smaller than the wavelength ofincident light at a location separated by a distance shorter than thewavelength of incident light from the Schottky junction composed of thesecond semiconductor layer and the conductive film.

In the photodiode of the present invention, a transparent film havingsubstantially the same index of refraction as the second semiconductorlayer may be provided on the first surface of the conductive film, andfurther, an antireflection film for the incident light may be provided.

The period of the periodic structure is preferably no greater than thewavelength of incident light, or is preferably set to the resonancewavelength of the surface plasmon excited on the conductive film by theincident light.

The thickness of the metal film is, for example, at least 100 nm but notgreater than 1000 nm in concave portions of the periodic structure, andthe depth of the surface irregularities in the periodic structure is,for example, at least 20 nm but not greater than 200 nm.

In the present invention, the planar shape of the aperture formed in theconductive film may be a circle, an oval, an ellipse, a dumbbell shapein which the central portion is constricted, or a slit. When theaperture is an oval, the minor axis of the oval should be made equal toor less than the wavelength of incident light. When a slit-shapedaperture is used, the distance between the two opposing sides of theslit should be equal to or less than the wavelength.

In the present invention, moreover, insulation material, for example, anoxide in film or cluster form having a diameter on the order of 2 nm orless, may be present in a range that does not impede the operation ofthe photodiode at the interface of the Schottky junction that is formedby the conductive film and the semiconductor layer.

The fabrication method of the photodiode of the present invention is afabrication method of a photodiode that includes: a conductive filmhaving an aperture and periodic surface irregularities that center onthe aperture; and a semiconductor layer that is joined to the conductivefilm at the bottom of the aperture; the fabrication method including thesteps of: defining and forming the semiconductor layer such that theregion for performing photoelectric conversion is limited to theposition that corresponds to the bottom of the aperture; forming theconductive film; and forming the aperture and the surface irregularitiesin the conductive film such that the aperture and the surfaceirregularities are matched to the region.

In general, light that is incident to a metal film having a minuteaperture equal to or less than the wavelength is nearly unable to passthrough the aperture. However, as described above, it is known that theintensity of transmitted light can be enhanced by providing a periodicsurface irregularities around the minute aperture and then coupling theincident light with the surface plasmon of the metal film to enable theproduction of a resonant state. This effect is referred to as the“plasmon enhancement.” However, according to the above-described paperof Tineke Thio et al. (Non-Patent Document 1), the total energy of lightthat passes through an aperture having a diameter of 40% or less of thewavelength is attenuated to 1% or less of the energy of the incidentlight, and a practical high SN ratio therefore cannot be obtained byirradiating a semiconductor with only the transmitted light from, aminute aperture.

On the other hand, the bleeding phenomenon of photons referred to asnear-field light (evanescent light) occurs in the vicinity of the outletof the minute aperture. Plasmon enhancement resulting from periodicsurface irregularities is believed to greatly enhance this near-fieldlight in addition to the transmitted light. As a result, the productionof strong near-field light that attenuates exponentially within a narrowrange of approximately 100 nm or less as the bleeding length (i.e., thedistance from the position of the outlet) can be expected in thevicinity of the outlet of the minute aperture, this near-field lightbeing in a range of the same order as the area of the aperture in planarspread. In this case, the minute aperture need not completely penetratethe metal film, and a metal layer on the order of 10 nm may remain atthe bottom of the aperture, the near-field light surpassing this metallayer and appearing on the outlet side. When an extremely thin metallayer remains, the outlet of the aperture is a position that correspondsto the position of the aperture on, of the surfaces of the metal layer,the surface that is not the bottom surface of the aperture. Thisnear-field light is influenced by the refractive index of material thatcontacts the metal film or periodic structure of the metal film, wherebythe range of bleeding and intensity of the near-field light varies, butthe near-field light appears even when a material such as asemiconductor exists on the outlet side. Near-field light localized inthe vicinity of a minute aperture is believed to be absorbed within asemiconductor similar to ordinary propagated light to produceelectron-hole pairs. The generation of electron-hole pairs based on thisnear-field light is carried out only in the localized near-field lightregion, and is added to the photocurrent that results from ordinarypropagated light. Accordingly, if the electron-hole pair generation fromthis near-field light is greater than the electron-hole pair generationresulting from propagated light, the majority of the energy of incidentlight can generate a photocurrent in the extremely narrow near-fieldlight region at a depth, of approximately 100 nm or less.

The inventors of the present invention have here fabricated a Schottkyphotodiode for detecting plasmon-enhanced near-field light and haveinvestigated the photocurrent when the incident light has a wavelengthof 830 nm. In the photodiode, a silver (Ag) film having a thickness of200 nm and having aperture with a diameter of 300 nm is formed on thesurface of a silicon (Si) substrate, the position of this aperture beingthe photodetection portion. Concentric grooves having a depth of 50 nmand a period of 560 nm are provided in the silver film around theaperture and the surface plasmon is excited by the irradiation ofincident light onto the silver film, whereby surface plasmon resonanceoccurs. A chromium (Cr) layer of a thickness of 10 nm is furtherinterposed at the interface between the silicon substrate and the silverfilm to realize close adherence of the substrate and the film. For thesake of comparison, a photodiode was also fabricated in which aconcentric periodic structure was not provided on the silver film.

FIG. 3 shows the relation observed between the reverse bias current andthe photocurrent of these photodiodes. In FIG, 3, dotted line A is theactually measured value of the photocurrent for the photodiode thatlacks the concentric periodic structure on the silver film, and dottedline B shows the predicted value resulting from a calculation of thephotocurrent in the photodiode having the concentric periodic structureon the silver film. The enhancement rate resulting from surface plasmonresonance is estimated for the intensity of transmitted light that haspassed through the aperture, and the predicted value is then estimatedbased on this enhancement rate. The enhancement rate resulting fromsurface plasmon resonance was here estimated as approximately 20 timesbased on the data of the above-described paper of Tineke Thio et al.(Non-Patent Document 1) and with the diameter of the aperture as 36% ofthe wavelength of the incident light. Solid line C shows the actuallymeasured value of photocurrent in the photodiode having a concentricperiodic structure on the silver film.

As can be clearly seen from the results shown in FIG. 3, when anaperture having a diameter smaller than the wavelength is formed in ametal film, the actually measured value of the photocurrent for a casein which a concentric periodic structure in the metal film is presentaround the aperture is greater than the value estimated from theenhancement rate for the transmitted light. In particular, it wasascertained that when the reverse bias voltage is low, the photocurrentthat flows is greater than the surface plasmon enhancement rate by afactor of 10. Based on the marked increase in photocurrent that exceedsexpectations when the reverse bias voltage is low, this phenomenonbecomes conspicuous when the depletion layer formed at the interfacebetween the silver film and semiconductor layer is thin and is believedto result from the photon field that concentrates in the vicinity ofthis type of interface. In other words, the cause of this largephotocurrent is believed to be the influence of the near-field lightthat was not estimated by Tineke Thio et al. Based on experimentationthe inventors of the present invention have discovered that when aminute aperture is used, the contribution of near-field light surpassesthe contribution of transmitted light to an extent that exceeds allexpectations.

Conventionally, it wad believed that the optical absorption coefficientof semiconductor material in which electron-hole pairs are generated dueto photons is relatively low, and therefore that electron-hole pairscould not be formed in sufficient numbers within the region in whichnear-field light is generated. In contrast, based on the results ofexperimentation by the inventors of the present invention, as shown inFIG. 3, it has been found that using near-field light that has beenplasmon-enhanced in the vicinity of a minute aperture enables theproduction of an extremely small photodiode having sufficiently highphotoelectric conversion efficiency.

In other words, adjusting the shape of a metal film on the semiconductorsubstrate and the carrier concentration in the semiconductor substratesuch that the region of near-field light and the region of the depletionlayer overlap enables the sufficient generation of electron-hole pairsby photons in the extremely narrow area of the degree of spread of thenear-field light. In this case, the area of the depletion layer or thearea of the junction may be on the order of the spread of near-fieldlight, and in addition, the thickness of the depletion layer may be onthe order of the bleeding of the near-field light, and as a result, thearea and thickness of the depletion layer can be made far smaller than aphotodiode of the prior art while maintaining high quantum efficiency. Aphotodiode can therefore be obtained that simultaneously realizes highquantum efficiency, high-speed response, and reduced dark current.

In the ordinary configuration of a photodetection circuit using aphotodiode, the product of the load resistance connected to thephotodiode for detecting signal voltage and the electrical capacitanceof the junction portion in the photodiode is substantially the timeconstant of the circuit, as shown in FIG. 1 or FIG. 2. The photodiode ofthe present invention enables a dramatic reduction of the electricalcapacitance by reducing the area of the junction, and to this extent,enables a reduction of the time constant of the circuit to realizehigh-speed operation.

When n-type silicon is used as the semiconductor, the length of thedepletion layer is approximately 100 nm if the concentration ofimpurities is set to 10¹⁷ cm⁻³, and the length of the depletion layer is300 nm or more if the concentration of impurities is set to 5×10¹⁵ cm⁻³.Since the relative permittivity (dielectric constant) of silicon isapproximately 12, setting the spacing of the pair of electrodes thatsandwich the Schottky junction to no more than the length of thedepletion layer, i.e., 100 nm, results in a junction capacitance of 0.1fF for a case in which a round Schottky junction with a diameter of 300nm is provided. Even if the parasitic capacitance accompanying thewiring pattern is assumed to be approximately 100 times the junctioncapacitance, the RC time constant of the photodiode circuit isapproximately 0.5 ps when the load resistance is 50 Ω, from which ahigh-speed response of 300 GHz or more can be expected.

On the other hand, the response frequency that is found from the carriertransit time for passing through a depletion layer of 100 nm when usinghighest drift speed of 10 ⁷ cm/s that can be expected in silicon isapproximately 160 GHz.

As can be seen from the above-described estimation, a circuit that usesthe photodiode of the present invention is capable of extremely highspeed response of 100 GHz or more even when using silicon, which has arelatively low absorption coefficient of light. In addition, if the sametime constant as a circuit that uses the photodiode of the prior art isacceptable, a load resistance that is increased to the extent of thereduction of the junction capacitance is possible, and a higher signalvoltage can therefore be obtained. Still further, the small volume ofthe depletion layer in the photodiode of the present invention decreasesthe incidence of noise that results from dark current.

The frequency range of light that can be detected using the photodiodeof the present invention is the region in which the energy of photons islower than the energy gap of the semiconductor, and moreover, equal toor greater than the plasma frequency of free electrons in the metalfilm. By selecting the materials used for the semiconductor and metalfilm, the shape of the surface periodic structure, and the diameter ofthe aperture of the metal film, the photodiode of the present inventioncan be used in the detection of light of the entire electromagnetic waveregion including visible light, near infrared light, and far infraredlight.

Effect of the Invention

As should be clear from the foregoing explanation, the present inventioncan obtain a photodiode in which the junction electrical capacitance isextremely small, and through the use of this photodiode, a photoelectricconversion circuit having high-speed response can be produced. Inaddition, the ability to connect a greater load resistance in a seriesto the photodiode allows a signal output voltage to be obtained that ishigher than that of the prior art. The ability to obtain a high signaloutput voltage in turn enables a reduction of the amplification rate ofthe amplifier that is provided in stages that follow the photoelectricconversion circuit or even allows the elimination of an amplifieraltogether, which not only allows the configuration of the photoelectricconversion circuit to be simplified to reduce fabrication costs but alsoallows the realization of a photoelectric conversion circuit having lowpower consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an example of the configuration of apin-type photodiode of the prior art;

FIG. 2 is a sectional view showing an example of the configuration of aSchottky photodiode of the prior art;

FIG. 3 is a graph showing the relation between reverse bias voltage andphotocurrent in a photodiode configured to detect near-field light thathas been enhanced by surface plasmon resonance;

FIG. 4 is a partial sectional perspective view showing the configurationof a Schottky photodiode according to a first embodiment of the presentinvention;

FIG. 5 is an enlarged sectional view showing the configuration of theSchottky photodiode of the first embodiment;

FIG. 6 is a plan view showing an example of the electrode arrangement ofthe Schottky photodiode of the first embodiment;

FIGS. 7A to 7J show the progression of fabrication processes of theSchottky photodiode of the first embodiment;

FIG. 8 is an enlarged sectional view showing a second example of theSchottky photodiode of the first embodiment;

FIG. 9 is a plan view of the electrode arrangement shown in FIG. 8;

FIG. 10 is a plan view showing a third example of the Schottkyphotodiode of the first embodiment;

FIG. 11 is an enlarged sectional view showing a fourth example of theSchottky photodiode of the first embodiment;

FIG. 12 is an enlarged sectional view showing a fifth example of theSchottky photodiode of the first embodiment;

FIG. 13 is an enlarged sectional view showing a sixth example of theSchottky photodiode of the first embodiment;

FIG. 14 is an enlarged sectional view showing a seventh example of aplanar Schottky photodiode of the first embodiment;

FIG. 15 is a sectional perspective view showing the configuration of aSchottky photodiode according to a second embodiment of the presentinvention;

FIG. 16 is an enlarged sectional view showing the configuration of aSchottky photodiode according to a third embodiment of the presentinvention;

FIG. 17 is an enlarged sectional view showing a second example of theSchottky photodiode of the third embodiment;

FIG. 18 is a schematic sectional view showing the configuration of anoptical reception module according to a fourth embodiment of the presentinvention; and

FIG. 19 is a schematic sectional view showing the configuration of anoptical interconnection module according to a fifth embodiment of thepresent invention.

EXPLANATION OF REFERENCE NUMERALS

-   1, 16, 24 substrate-   2, 14, 25, 60 n⁺-type semiconductor layer-   3, 15, 26, 61 n⁻-type semiconductor layer-   4, 17 metal periodic structure member-   5, 19, 29, 54, 62 first electrode-   8, 20, 31, 55, 63 second electrode-   6, 18, 27 aperture-   7, 30 insulating layer-   9, 21, 32, 56, 64 bias power supply-   10, 22, 33, 50, 65; load resistance-   11, 23, 57, 67 incident light-   12 slit-   13 antireflection film-   28 metal structure member-   40 scattering member-   51 i-layer-   52 p-layer-   53 n-layer-   59, 69 window-   66 semi-transparent metal film

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Explanation next regards a Schottky photodiode according to a firstembodiment of the present invention. The Schottky photodiode of thefirst embodiment is configured as a planar photodiode. FIG. 4 is apartial sectional perspective view showing the overall configuration ofthis photodiode, and FIG. 5 shows an enlargement of this sectional view.

The Schottky photodiode shown in the figures is provided with: n⁺-typesemiconductor layer 2 formed on a portion of substrate 1 such as SOI(Silicon-On-Insulator) in which the surface is insulating; n⁻-typesemiconductor layer 3 provided on a portion of n⁺-type semiconductorlayer 2; metal periodic structure member 4 that contacts n⁻-typesemiconductor layer 3 and that has electrical conductivity; firstelectrode (anode electrode) 5 connected to metal periodic structuremember 4; second electrode (cathode electrode) 8 that confronts firstelectrode 5 and contacts n⁺-type semiconductor layer 2; bias powersupply 9 for applying a reverse bias voltage across first and secondelectrodes 5 and 8; and load resistance 10 inserted between bias powersupply 9 and first electrode 5.

Aperture 6 is provided in metal periodic structure member 4. As will belater explained, aperture 6 does not fully penetrate metal periodicstructure member 4. Metal periodic structure member 4 contacts n⁻-typesemiconductor layer 3 only at aperture 6 and positions in the vicinityof aperture 6, and further, n⁻-type semiconductor layer 3 is formed onn⁺-type semiconductor layer 2 only at aperture 6 and the positions inthe vicinity of aperture 6. In other locations, insulating layer 7 isprovided between metal periodic structure member 4 and n⁺-typesemiconductor layer 2 or substrate 1. First electrode 5 is provided oninsulating layer 7 and connects to metal periodic structure member 4.Second electrode 8 is also provided on insulating layer 7. A portion ofn⁺-type semiconductor layer 2 penetrates insulating layer 7 and appearsat the upper surface of insulating layer 7, and n⁺-type semiconductorlayer 2 is joined to second electrode 8 in this location.

In this configuration, incident light 11 is incident to metal periodicstructure member 4 which is formed as a metal film through which lightsubstantially does not pass. Incident light 11 excites the surfaceplasmon in metal periodic structure member 4 and couples with thissurface plasmon to produce a resonant state with the surface plasmon bymeans of the periodic structure. The generation of this resonant stategenerates strong near-field light in the vicinity of the semiconductorsurface on the opposite surface of the light-incident surface ofaperture 6 that is located in the center of metal periodic structuremember 4. This near-field light generates electron-hole pairs in thedepletion layer that accompanies the Schottky barrier on thesemiconductor side of the vicinity of aperture 6 and thus generatesphotovoltaic energy. The intensity of incident light 11 is converted tothe voltage difference over the both ends of load resistance 10.

As described in the foregoing explanation, the semiconductor portionbelow metal periodic structure member 4 is made up from substrate 1,n⁺-type semiconductor layer 2 having high electric conductivity, andn⁻-type semiconductor layer 3 that is formed on a portion of n⁺-typesemiconductor layer 2. N⁺-type semiconductor layer 2 is obtained byimplanting impurity ions at a concentration of at least 1×10²⁰ cm⁻³ intosilicon. N⁻-type semiconductor layer 2 can be formed by epitaxial growthof semiconductor crystal containing impurity ions on the order of 1×10¹⁷cm⁻³ on n⁺-type semiconductor layer 2. A more specific explanationregarding the fabrication processes will be given later.

Metal periodic structure member 4 is formed from a material such assilver (Ag) or gold (Au) having little plasmon loss. Aperture 6 isprovided in the central portion of metal periodic structure member 4,and a periodic structure composed of concentric surface irregularitiesis formed in metal periodic structure member 4 with this aperture as acenter. The period of the surface irregularities of metal periodicstructure member 4 is the resonance wavelength of the surface plasmonand is set to a value slightly shorter than the period of the incidentlight, i.e., the signal light. For example, in the case of lightwavelength of 800 nm, the period of the surface irregularity is on theorder of 600 nm. The effect of surface plasmon resonance is exhibitedwhen the depth of the surface irregularity is set to a value on theorder of 20 to 200 nm. The film thickness of metal periodic structuremember 4 is preferably set to at least 100 nm such that virtually nolight is transmitted even at depressions where the film thickness is aminimum, and because excessive film thickness weakens the near-fieldlight in the vicinity of aperture 6, the film thickness must be nogreater than 1000 nm. First electrode 5 is connected to metal periodicstructure member 4 over insulating layer 7 and may be formed at the sametime as metal periodic structure member 4 using the same material asmetal periodic structure member 4.

The shape of the surface irregularity on metal periodic structure member4 is not limited to the rectangular profile shown in FIG. 2 and may alsoassume a shape in which the apices of the ridges have a triangularprofile, the side surfaces have a curved surface, or the corners arerounded. Even with these shapes, the same effect produced by surfaceplasmon resonance is seen. In addition, the ratio of the length of thedepressions and the length of the ridges can be any value. The effectcan be seen to some degree even when only one period of concentricirregularity is provided, i.e., when only a single circular structure isformed surrounding aperture 6. However, a greater surface plasmonresonance effect can be obtained by providing three or more periods,i.e., triple or higher concentric circular structure. Incident light 11is irradiated onto a portion of, or over all of metal periodic structuremember 4.

The area over which n⁻-type semiconductor layer 3 contacts metalperiodic structure 4 is set to substantially overlap the region ofaperture 6. The region of generation of the near-field light thatappears in the vicinity of the minute aperture usually spreads somewhatbeyond the diameter of aperture 6. As a result, the junction areabetween metal periodic structure member 4 and n⁻-type semiconductorlayer is normally set larger than the diameter of aperture 6 by thelithography positioning error, i.e., on the order of 10 nm to 500 nm oneach side. In contrast, operation as a photodiode is possible even ifaperture 6 is formed protruding beyond the junction area such that aportion of the near-field light appears outside n⁻-type semiconductorlayer 3.

Aperture 6 does not completely penetrate metal periodic structure member4, and a metal layer having a thickness of approximately 10 nm or lessremains at the bottom of aperture 6. This metal layer is composed of amaterial having little surface plasmon loss such as silver or gold,which is the material that forms metal periodic structure member 4, oris an adhesion layer such as chromium (Cr), or is a laminated structureof both materials. Alternatively, this metal layer may also be anincomplete film in which holes such as pinholes have been formed inportions. A Schottky barrier occurs along the interface between metalperiodic structure member 4 and n⁻-type semiconductor layer 3. Thepropagation distance of near-field light is 100 nm or less, and theregion of greatest intensity is localized at a position at a depth ofapproximately 10 nm in n⁻-type semiconductor layer 3. The calculatedwidth of the depletion layer brought about by the Schottky barrier is onthe order of 100 nm in the case of silicon in which the donor impurityconcentration is 1×10¹⁷ cm⁻³, and therefore the thickness of the n⁻-typesemiconductor layer 3 can be reduced to 50 nm or less by furtherincreasing the donor impurity concentration.

N⁺-type semiconductor layer 2 has a doping concentration of impurity ashigh as 1×10²⁰ cm⁻³ or more and makes ohmic contact with the secondelectrode. However, even when the doping concentration in n⁺-typesemiconductor layer 2 is slightly lower than 1×10²⁰ cm⁻³, photodiodeoperation can be obtained that is similar to that of the above-describedconfiguration if the Schottky barrier height that is formed betweenn⁺-type semiconductor layer 2 and the second electrode is relativelylower than the Schottky barrier between metal periodic structure member4 and n⁻-type semiconductor layer 3

First electrode 5 and second electrode 8 are connected to loadresistance 10 by way of bias power supply 9, whereby the intensity ofincident light 11 is converted to the difference in potential across theboth ends of load resistance 10 in this circuit configuration.

FIG. 6 shows an example of the positional relations of metal periodicstructure member 4, first electrode 5, and second electrode 8 oninsulating layer 7. In the example shown here, first electrode 5 andsecond electrode 8 exist on the same plane, and second electrode 8 isarranged in a shape that intrudes into an area notched into a portion ofthe concentric surface irregularities of metal periodic structure member4. However, first electrode 5 and second electrode 8 need not be formedon the same plane. In addition, because second electrode 8 need notbring about plasmon enhancement, second electrode 8 does not require theuse of the same material such as gold or silver as first electrode 5 andcan use, for example, copper (Cu), nickel (Ni), tungsten (W), oraluminum (Al).

When the shape of aperture 6 is a circle, the conspicuous enhancement ofnear-field light brought about by the surface plasmon occurs when thediameter of aperture 6 is within a range from one-half to 1/10 thewavelength of incident light. This is because the greater part of thephoton energy is transmitted through aperture 6 as propagated light andthe energy therefore cannot be confined as near-field light whenaperture 6 is on the same order as the wavelength or larger than thewavelength. On the other hand, when the diameter of aperture 6 issmaller than 1/10 the wavelength of the incident light, there is littlepropagation of energy to the opposite side of aperture 6 despite the useof plasmon enhancement, and the greater part of the photon energy isreflected. Accordingly, the diameter of the aperture is preferably onthe order of 80 nm to 400 nm when using incident light having awavelength of 800 nm. In addition to a circle, aperture 6 can also takethe form of a rectangle, hexagon, or an oval.

As described in the foregoing explanation, metal periodic structuremember 4 contacts n⁻-type semiconductor layer 3 only in the vicinity ofaperture 6, and in other locations, insulating layer 7 is presentbetween metal periodic structure member 4 and each semiconductor layer.This is because the depletion layer existing in the range not reached bylight makes no contribution to photovoltaic energy and contributes onlyto dark current. A material used in ordinary semiconductor processessuch as SiO₂ may be used as insulating layer 7. In addition, when metalperiodic structure member 4 and n⁻-type semiconductor layer 3 formunwanted parasitic capacitance outside the depletion layer and increasethe overall electrical capacitance, the response speed of the circuit isdelayed. Thus, insulating layer 7 must be set to the greatest possiblethickness in order to reduce the dark current and increase the responsespeed. Insulating layer 7 is preferably set to a thickness of at least500 nm, whereby a reduction of the parasitic capacitance to a value of 1fF or less can be expected. An insulating film having a low dielectricconstant such as SiOF or SiOC obtained by doping SiO₂ with fluorine (F)or carbon (C) can also be used as the material of insulating layer 7,these materials being effective for reducing parasitic capacitance.

<<Fabrication Method>>

Explanation next regards a method of fabricating the above-describedphotodiode when silicon is used as the semiconductor material. FIGS. 7Ato 7J show the progression of the fabrication method of the photodiodeof the first embodiment.

First, as shown in FIG. 7A, a substrate is prepared in which n⁺-typesemiconductor layer 2 is formed over the entire surface of one principlesurface of substrate layer 1 composed of, for example, SiO₂, and n⁻-typesemiconductor layer 3 is formed over the entire surface of n⁺-typesemiconductor layer 2. N⁺-type semiconductor layer 2 is obtained byimplanting phosphorus (P) ions into a silicon layer at a concentrationof at least 1×10²⁰ cm⁻³ to achieve electrical resistivity on the orderof 10⁻⁴ Ω.cm. N⁻-type semiconductor layer 3 is obtained by depositing ann-type silicon that contains phosphorus on the order of 1×10¹⁷ cm⁻³ onn⁺-type semiconductor layer 2 by a chemical vapor method to a thicknessof approximately 100 nm.

Next, as shown in FIG. 7B, mask 701 is formed on n⁻-type semiconductorlayer 3 aligned with the area in which n⁺-type semiconductor layer 2 isto be formed in the completed photodiode. Mask 701 is obtained byproviding a photoresist of a desired shape on silicon nitride having athickness of 100 nm to 150 nm deposited by, for example, a chemicalvapor method or the like, and then subjecting the silicon nitride to ionetching or the like. Next, as shown in FIG. 7C, mask 701 is used to etchn⁻-type semiconductor layer 3 and n⁺-type semiconductor layer 2. At thistime, in areas in which mask 701 is not provided, n⁻-type semiconductorlayer 3 should be entirely removed and the greater part of n⁺-typesemiconductor layer 2 should be remove to reduce the thickness thereof.Chemical dry etching, which is well known in semiconductor fabricationprocessing, is used as the etching method, and CF₄ or a gas mixture ofCF₄ and O₂ is used as the reactive gas.

Next, as shown in FIG. 7D, mask 701 is further patterned to produce mask702 of approximately 1 μm diameter for Schottky connection correspondingto aperture 6 and mask 703 that is larger than mask 702 for ohmicconnection of n⁺-type semiconductor layer 2. Masks 702 and 703 areformed by ion etching or the like. Masks 701 and 702 are next used, asshown in FIG. 7E, in the etching of n⁻-type semiconductor layer 3 andn⁺-type semiconductor layer 2 to form mesa 704 for Schottky connectionand mesa 705 for ohmic connection, whereby n⁺-type semiconductor layer 2and n⁻-type semiconductor layer 3 attain the same shape as in thecompleted photodiode. In other words, n⁺-type semiconductor layer 2 isentirely removed from areas other than the area of mesa 704 for Schottkyconnection, the area of mesa 705 for ohmic connection, and the area thatjoins the two mesas 704 and 705. Chemical dry etching, which is wellknown in semiconductor fabrication processing, is used as the etchingmethod, and CF₄ or a gas mixture of CF₄ and O₂ is used as the reactivegas.

N⁻-type semiconductor layer 3 and n⁺-type semiconductor layer 2 are nextembedded in insulating layer 7. Insulating layer 7 is composed of, forexample, SiO₂, and is formed by bias CVD, which is a plasma chemicalvapor method in which ions are drawn to the substrate side to bury thedifferences in levels. After the SiO₂ film has been grown, the substrateis placed in hot phosphoric acid at approximately 130° C. for about onehour to remove masks 702 and 703. FIG. 7F shows the substrate followingthe removal of masks 702 and 703. A substantially flat surface can beobtained by optimizing the mesa shape and oxidation processes, but amore even surface can be obtained by using a polishing technique such asmechano-chemical polishing (CMP).

Next, as shown in FIG. 7G, n⁻-type semiconductor layer 3 on mesa 705 forohmic connection is removed by means of, for example, chemical ionetching. Metal layer pattern 706 is next provided in the positions thatare to become metal periodic structure member 4, first electrode 5, andsecond electrode 8 as shown in FIG. 7H. This metal layer pattern 706 ispreferably composed of a material having low electrical resistivity. Inaddition, the film thickness of this metal layer pattern 706 ispreferably no greater than 200 nm to keep the transmittance of thesignal light at aperture 6 of metal periodic structure member 4 at ahigh level, and to prevent interference of the enhancement effectproduced by the surface plasmon. When material such as silver which haspoor adhesion to silicon is used for metal layer pattern 706, chromium,titanium, tantalum, tungsten, nickel or the like provided as an adhesionlayer at a thickness of 10 nm or less and preferably 3 nm or less can beprovided as a foundation layer for metal layer pattern 706. Metal layerpattern 706 is formed divided between a pattern for covering the entireSchottky connection mesa and a pattern for covering a portion of theohmic connection mesa, the two patterns being electrically separated.

Aperture 6 is next formed in metal layer pattern 706 at a position inthe central portion of mesa 704 for Schottky connection as shown in FIG.71. A film is further formed for metal periodic structure member 4,first electrode 5, and second electrode 8 on metal layer-pattern 706. Atthis time, a photoresist mask is provided in advance such that the metalfilm is not formed in areas other than the positions of formation ofmetal layer pattern 706. After forming the electrodes, the well-knownlift-off method is implemented for removing the photoresist masktogether with the unneeded metal, whereby metal periodic structuremember 4, first electrode 5, and second electrode 8 are formed. Amaterial such as silver or gold having low electrical resistance in theoptical region and that limits surface plasmon loss is preferably usedfor the metal periodic structure member and the electrodes, but secondelectrode 8 need only be a conductor and does not require the use ofsilver or gold.

Next, as shown in FIG. 7J, periodic surface irregularities with pitch Pis formed in the surface of metal periodic structure member 4 by, forexample, additionally forming the same metal as the electrodes (silveror gold) as additional metal layer 707 by a lift-off method or the like.

The photodiode shown in FIGS. 4 to 6 is completed by the above-describedprocesses.

In the first embodiment described above, another semiconductor of the IVgroup elements such as germanium or SiGe can be used in place of siliconas the semiconductor material for making up the photodiode. A III-Vgroup compound semiconductor of GaAs or InP can also be used. Theselection of these materials is determined based on the wavelengthlimits that are restricted by the band gaps and absorption coefficientsof light. Superior photodiode characteristics are obtained atwavelengths in the vicinity of 800 nm to 900 nm when silicon is used, atwavelengths in the vicinity of 1300 nm to 1500 nm when germanium isused; and at wavelengths in the vicinity of 1300 nm to 1600 nm whenInGaAs grown on InP is used. When germanium is used, a substrate such asa GOI (Germanium On Insulator) substrate that has been fabricated usingan ultra-high vacuum chemical vapor deposition method, molecular beamepitaxial method or the like is used in place of an SOI substrate. Whena III-V group compound semiconductor is used to form the photodiode, asemi-insulating substrate may be used in place of an insulatingsubstrate, and an n⁺-type III-V group compound semiconductor layer canbe formed on the semi-insulating substrate. For example, an n⁺-type GaAslayer is formed on a GaAs substrate, which is a semi-insulatingsemiconductor, following which an n⁻-type GaAs layer is further formedon a portion of this surface. A metal periodic structure member madefrom silver is arranged in contact with this n⁻-type GaAs layer.

Explanation next regards another example of the configuration of thephotodiode of the above-described first embodiment.

FIG. 8 and FIG. 9 show a second example of the Schottky photodiode ofthe first embodiment. This photodiode is similar to the photodiode shownin FIGS. 4 to 6, but differs from the photodiode shown in FIGS. 4 to 6in that a partial notch portion for the encroaching provision of secondelectrode 8 is not provided in metal periodic structure member 4 thathas concentric surface irregularities.

FIG. 10 shows a third example of the Schottky photodiode of the firstembodiment. This photodiode is similar to the photodiode shown in FIGS.4 to 6, but differs from the photodiode shown in FIGS. 4 to 6 in thatmetal periodic structure member 4 has a shape in which the metalperiodic structure member in the photodiode shown in FIGS. 4 to 6 is cutinto a strip form and second electrode 8 is formed along the three sidesof metal periodic structure member 4 so as to surround this strip-shapedmetal periodic structure member 4.

FIG. 11 shows a fourth example of the Schottky photodiode of the firstembodiment. This photodiode is similar to the photodiode shown in FIG.10, but differs from the photodiode shown in FIG. 10 in that narrow slit12 is formed in place of aperture 6, and the surface irregularities ofthe metal periodic structure member are not formed as concentric surfaceirregularities but as parallel grooves. The gap of slit 12 is preferablyone half or less the wavelength of incident light, and the longitudinallength of slit 12 is preferably no greater than the wavelength of theincident light.

FIG. 12 shows a fifth example of the Schottky photodiode of the firstembodiment. This photodiode has substantially the same construction asthe photodiode shown in FIGS. 4 to 6, but differs in that the interfaceat which metal periodic structure member 4 contacts n⁻-typesemiconductor layer 3 and the interface at which metal periodicstructure member 4 contacts insulating layer 7 are on substantially thesame plane. Here, “substantially the same plane” means that thedifference in levels between the two interfaces is sufficiently smallwith respect to the wavelength of light, and more specifically, adimension of 1/30 the wavelength or less. A photodiode having thisconstruction can be fabricated by, when forming the mesa for Schottkyconnection and the mesa for ohmic connection, using different masks forthe position on n⁺-type semiconductor layer 2 that contacts the firstelectrode and the position that contacts the second electrode to depositn⁻-type semiconductor layer and n⁺-type semiconductor layer,respectively, by molecular beam epitaxy or ultra-high vacuum chemicalvapor method. This construction has the advantage of facilitating thesimultaneous formation of metal periodic structure member 4, firstelectrode 5, and second electrode 8.

FIG. 13 shows a sixth example of the Schottky photodiode of the firstembodiment. In this photodiode, in contrast to the photodiodes each ofthe above-described examples, aperture 6 that is provided on metalperiodic structure member 4 completely penetrates, whereby n⁻-typesemiconductor layer 3 below aperture 6 is exposed and metal periodicstructure member 4 contacts n⁻-type semiconductor layer 3 at theperiphery of the aperture. If the diameter of aperture 6 is no greaterthan one-half the wavelength of the incident light, the amount of lighttransmitted by aperture 6 is extremely small and nearly all photons thatappear on the opposite of aperture 6 will be in the form of near-fieldlight. Accordingly, a photodiode can be obtained that operates similarto a case in which a metal layer exists at the bottom of aperture 6.

FIG. 14 shows a seventh example of the Schottky photodiode of the firstembodiment. This photodiode is similar to the photodiode shown in FIG.13 but differs in that transparent film 14 having a refractive indexsubstantially equal to that of n⁻-type semiconductor layer 3 is providedon metal periodic structure member 4 to cover the entire periodicstructure of the surface of metal periodic structure member 4. Theprovision of transparent film 14 decreases the reflection of light onthe outlet side of aperture 6. For example, when a photodiode isconstructed for signal light having a wavelength of 1300 nm, germaniumthat has been doped with antimony is used as n⁻-type semiconductor layer3 and silicon or SiGe is used as transparent film 14 to realize thisconfiguration. In the configuration shown in FIG. 14, transparent film14 is covered by antireflection film 13 to obtain a further reduction ofreflection. A material having a refractive index close to the squareroot of the refractive index of transparent film 14 is preferably usedfor antireflection film 13. When silicon is used as transparent film 14,HfO₂ or Ta₂Or can be used for antireflection film 13.

Second Embodiment

Explanation next regards the Schottky photodiode according to the secondembodiment of the present invention. The photodiode of the firstembodiment was of the planar type, but the photodiode of this secondembodiment uses an insulating substrate and arranges the first electrodeon the front surface of the substrate and the second electrode on therear surface. FIG. 15 shows the configuration of a photodiode that isthe second embodiment.

As in the case of the first embodiment, on the surface of insulatingsubstrate 16, aperture 18 is provided at the center and metal periodicstructure member 17 having a periodic structure realized by concentricsurface irregularities is formed around aperture 18. As with thephotodiode shown in FIGS. 4 to 6, aperture 18 does not completelypenetrate metal periodic structure member 17. First electrode (anodeelectrode) 19 that is electrically connected to metal periodic structuremember 17 is further provided on the surface of substrate 16.

A through-hole is formed in substrate 16 corresponding to the positionof aperture 16, and second electrode (cathode electrode) 20 is providedon the surface on the opposite side of substrate 16 with thisthrough-hole interposed. N⁺-type semiconductor layer 14 for ohmicconjunction with second electrode 20 and n⁻-type semiconductor layer 15that is formed on n⁺-type semiconductor layer 14 are provided in thethrough-hole formed in substrate 16. N⁻-type semiconductor layer 15reaches the surface of substrate 16 and connects with metal periodicstructure member 17.

In this photodiode, the configurations and shapes described in thesecond to seventh examples in the first embodiment can be used for metalperiodic structure member 17 and each of electrodes 19 and 20.

First electrode 19 and second electrode 20 are connected to bias powersupply 21 by way of load resistance 22, and a reverse bias voltage isapplied to this photodiode from bias power supply 21.

When silicon is used as the semiconductor material in this photodiode,n⁺-type semiconductor layer 14 and n⁻-type semiconductor layer 15 areformed by adjusting the arsenic that is doped in the silicon. Silver canbe used for electrodes 19 and 20. In this photodiode as well, settingthe period of the concentric surface irregularities of metal periodicstructure member 17 to 700 nm produces conspicuous plasmon resonancewhen the wavelength of incident signal light is in the neighborhood of800 nm, and the intensity of near-field light that appears from thevicinity of aperture 18 on the side of n⁻-type semiconductor layer 15 isboth dramatically increased and closely confined to the vicinity of theoutlet side of aperture 18. As a result, a photodiode that exhibits highquantum efficiency can be obtained in the second embodiment as in thecase of the first embodiment.

Third Embodiment

Explanation next regards the Schottky photodiode according to the secondembodiment of the present invention. The photodiode of the thirdembodiment shown in FIG. 16 is similar to the photodiode of the firstembodiment but differs in that minute scattering member 40 forscattering light is provided on the bottom surface of aperture 6provided in metal periodic structure member 4. As the material for thisscattering member 40, a material in which the electrical resistance islow in the region of the wavelength of light is appropriate, and thesame metal material as used for metal periodic structure member 4, forexample, silver, gold or the like, can be used. The volume and shape ofscattering member 40 has an influence on the state of surface plasmonresonance, scattering members 40 of sizes ranging from about 5 nm squareto the same order as the wavelength of incident light exhibiting a broadrange of effects. Scattering member 40 can have, for example, acylindrical or square rod shape.

Providing scattering member 40 on the bottom surface of aperture 6enables a decrease of the components of propagated light on the outletside of aperture 6 while increasing the diameter of aperture 6 toincrease the near-field light. More specifically, aperture 6 can beincreased in size within a range in which the distance betweenscattering member 40 and the inner walls of aperture 6 does not exceedhalf the wavelength of the incident light. In addition, scatteringmember 40 and the bottom surface of aperture 6 need not be in contactand may be separated by a minute distance no greater than the wavelengthof incident light. In the device shown in the figure, scattering member40 is arranged on the metal film that makes up the bottom surface ofaperture 6 with thin dielectric layer 41 interposed. Alternatively, theaperture may completely penetrate the conductor, and the mode of thepenetrating aperture can also be applied in the configuration shown inFIG. 13.

FIG. 17 shows a second example of the photodiode in the thirdembodiment. This photodiode differs from the photodiode shown in FIG. 16in that minute scattering member 40 is embedded in the uppermost surfaceof n⁻-type semiconductor layer 3. A material having a low electricalresistance in the region of the wavelength of light is again appropriatefor this scattering member 40, and the same metal material such assilver or gold that was used in metal periodic structure member 4 can beused. In this example as well, the volume and shape of scattering member40 affects the state of a surface plasmon resonance, and scatteringmember 40 varying in size from about 5 nm square to the same order asthe wavelength of incident light exhibits effects over a broad range.When scattering member 40 is provided in n⁻-type semiconductor layer 3in contact with the metal layer of the bottom surface of aperture 6 inthis photodiode, the components of propagated light are decreased and astronger near-field light can be produced in the semiconductor layer inthe vicinity of the Schottky junction.

The same effects as previously described can also be obtained for aphotodiode of a configuration in which aperture 6 completely penetratesmetal periodic structure member 4 by arranging scattering member 40 inthis aperture 6 or by embedding scattering member 40 in the uppermostsurface of n⁻-type semiconductor layer 3.

Fourth Embodiment

Explanation next regards an example of the application of the Schottkyphotodiode of the present invention. FIG. 18 shows a photoreceptionmodule for 40 Gbps (gigabits per second) transmission that uses theSchottky photodiode of the present invention.

Optical fiber 73 is led from the outside into module case 78. In modulecase 78, photodiode 71 based on the present invention is arranged toconfront the end surface of optical fiber 73, and lens 74 for opticallycoupling optical fiber 73 and photodiode 71 and focusing signal light 77emitted from optical fiber 73 upon the photoreception surface ofphotodiode 71 is provided between the end surface of optical fiber 73and photodiode 71. Photodiode 71 is provided on the side surface of chipcarrier 72 and is connected by way of electrical wiring 76 topreamplifier IC (integrated circuit) 75 that is provided on the uppersurface of chip carrier 72. Photodiode 71 converts signal light 77 toelectrical signals, and supplies the electrical signals to preamplifierIC 75 by way of electrical wiring 76. Preamplifier IC 75 amplifies theelectrical signals that have been applied as input.

Photodiode 71 is formed by using a substrate in which an InGaAs film isformed on InP using epitaxial growth, and has a metal periodic structuremember composed of silver or gold on the InGaAs film. When thisphotodiode is used in transmission by infrared light having a wavelengthof 1.55 μm, the period of the surface irregularities on the metalperiodic structure member may be set to approximately 1.2 μm, and wheneight periods of concentric ring-shaped surface irregularities areformed, the diameter of the outer circumference is approximately 20 μm.The depth of the surface irregularities in the metal periodic structuremember is preferably on the order of 0.1 to 0.4 μm, and the diameter ofthe aperture is preferably on the order of 0.3 to 0.7 μm.

In a photoreception module for 40 Gbps transmission of the prior art, aphotodiode of a side-face incident waveguide type is frequently employedas the photodiode that is mounted inside the module case. The reason forthis is that a high absorption efficiency cannot be obtained when thethickness of the absorption layer is reduced for decreasing the chargecarrier transit time in a surface-incident type of photodiode in whichlight is irradiated on a semiconductor surface. On the other hand, byabsorbing light in an in-plane direction of the absorption layer, thewaveguide type obtains a high absorption efficiency with a short chargecarrier transit time. However, in waveguide-type device for 40 Gbps, thethickness of the absorption layer is normally 1 μm or less, and thecoupling tolerance relating to the alignment of the positions of thephotodiode and optical fiber must be on the order of ±1 μm. Aphotoreception module that uses a photodiode of the prior art thereforeentails serious problems from the standpoints of both package design andfabrication costs.

In contrast, a photodiode according to the present invention has aneffective diameter of 20 μm at the light-receiving surface, andtherefore, the coupling tolerance can be ±2 μm or more. As a result, theoptical coupling of the optical fiber and photodiode can be carried outby simple lens coupling, and costs can therefore be cut for thephotoreception module for transmission. In the photoreception module for40 Gbps transmission shown in FIG. 18, a minimum reception sensitivityof −12 dBm was obtained when transmitting a wavelength of 1.55 μm. Itwas confirmed that, by using the photodiode of the present invention, aphotoreception module could be realized at a level that, in terms ofcharacteristics, compared favorably with a photoreception module for 40Gbps of the prior art in which a waveguide-type photodiode was mounted.

Fifth Embodiment

Explanation next regards another example of the application of theSchottky photodiode of the present invention. FIG. 19 shows an opticalinterconnection module for connection between LSI (Large-ScaleIntegration) chips in which the Schottky photodiode of the presentinvention is mounted.

The interconnection of LSI by optical fiber and the transmission ofsignals as optical signals are being investigated as a means oftransmitting signals at high speed between LSI that are mounted on awiring board. Signal processing within LSI is carried out for electricalsignals and an optical interconnection module is therefore required forrealizing connection between optical fibers and each of the LSI chips,converting signal light from optical fibers to electrical signals forinput to an LSI chip, and for converting electrical signals suppliedfrom LSI chips to light signals and introducing these signals to fibers.

Photodiode 81 based on the present invention and VCSEL (Vertical CavitySurface Emitting Laser) light source 82 provided with electricalmodulation structure are provided on one surface of mounting board 89,and metal periodic structure member 90 for enhancing the intensity ofthe near-field light by means of surface plasmon resonance is formed onthe light-receiving surface of photodiode 81. Mounting board 89 isattached to the surface of LSI package 87 that incorporates an LSI chip.Vias 85 for the electrical wiring for the light source and modulationand vias 86 for the electrical wiring for the photodiode are formed inLSI package 87. Vias 85 are formed in mounting board 89 and areconnected to electrical wiring layer 91 that is connected to VCSEL lightsource 82. Vias 86 are formed in mounting board 89 and are connected toelectrical wiring layer 92 connected to photodiode 81.

LSI mounting board 88 is arranged to confront this mounting board 89.The surface of LSI mounting board 88 is provided with: optical fiber 83for optical signal input, optical fiber 84 for optical signal output,concave mirror 93 for directing signal light emitted from the endsurface of optical fiber 83 toward photodiode 81, and concave mirror 94for directing signal light from VCSEL light source 82 toward opticalfiber 84. Concave mirror 93 optically couples optical fiber 83 andphotodiode 81, and concave mirror 94 optically couples optical fiber 84and VCSEL light source 82.

In this type of optical interconnection module, signal light from fiber83 for optical signal input is irradiated onto metal periodic structuremember 90 by concave mirror 93. When light having a wavelength of 850 nmis used as the signal light, silicon is used as the semiconductormaterial used in photodiode 81 and the period of the surfaceirregularities in metal periodic structure member 90 is set to 600 nm to700 nm. Photodiode 81 made from silicon generates a photocurrent bymeans of the near-field light that is produced by metal periodicstructure member 90 and sends a current that corresponds to the opticalsignals to the LSI through vias 86 and electrical wiring layer 92 forthe photodiode. By means of the function of metal periodic structuremember 90, the coupling tolerance relating to the positions of concavemirror 93 and the photodiode can be set to ±1 μm or more. In this case,a preamplifier for amplifying the electrical signals can be providedmidway through electrical wiring layer 92 at a position that immediatelyfollows photodiode 81.

Electrical signals from LSI pass from vias 85 and through electricalwiring layer 91 to be converted to optical signals by VCSEL light source82 that is provided with an electrical modulation structure. The opticalsignals are reflected by concave mirror 94 and thus introduced tooptical fiber 84 for optical signals. VCSEL light source 82 providedwith the electrical modulation structure can be replaced by anotherknown structure for modulating light by electrical signals such as aMach-Zehnder modulator for modulating light from an outside light sourceby an electrooptic effect or by a thermooptic effect.

Another well-known construction such as a planar optical waveguide canbe used in place of optical fiber for the input of optical signals inthe optical interconnection module described in the foregoingexplanation. In addition, a condensing structure such as a convex lenscan also be used in place of concave mirror 93.

When high-speed operation of 20 GHz or more is the object in an opticalinterconnection module of the prior art used for connection between LSIchips, a compound semiconductor material such as InGaAs grown on an InPsubstrate is used to achieve high-speed response as the photodiode forphotoreception. This type of compound semiconductor is difficult tomatch to the fabrication processes of silicon semiconductor devices, andas a result, optical interconnection modules of the prior art entailedthe problem of high fabrication costs.

In contrast, an optical interconnection module of the present embodimentuses a photodiode according to the present invention that uses siliconas the semiconductor material, and fabrication costs can therefore bereduced. It has been confirmed that when the optical interconnectionmodule shown in FIG. 19 was actually fabricated, high-speedoptoelectrical conversion operation of approximately 40 GHz wasrealized.

Other Embodiments

Although the foregoing explanation has regarded preferable embodimentsof the present invention, it will be obvious that in each of theabove-described embodiments, technology well known in the prior art canbe combined with the techniques of the present invention such as thecondensing of incident light by a lens and the subsequent irradiation ofthe light onto a metal periodic structure member.

The planar shape of the aperture provided in metal periodic structuremember is not necessarily limited to a circle or a slit, and can also bean oval, an ellipse, a dumbbell, or even a square or rectangle, and thesame effects as described above can be obtained even when an aperture ofthese shapes is used. A plurality of apertures arranged in proximity canalso be used in place of a single aperture.

Regarding the shape along the direction of depth of the aperture, theabove-described embodiments included a configuration in which theaperture does not penetrate the metal periodic structure member and ametal layer remains on the bottom surface of the aperture, aconfiguration having a conductive optical scattering member on thebottom surface of the aperture, and a configuration in which the metalperiodic structure member is completely penetrated and the aperture hasno bottom surface. However, the sectional shape of the aperture is notlimited to these forms, and apertures of various other sectional shapescan be selected in each of the above-described examples. Accordingly,any combination of the planar shape and sectional shape of the apertureis possible including the forms shown in the examples.

1. A photodiode comprising: a conductive film having: an aperture havinga diameter smaller than wavelength of incident light, and a periodicstructure provided around said aperture for producing a resonant stateby an excited surface plasmon in a film surface of said conductive filmby means of the incident light to said film surface; and a semiconductorlayer provided in a vicinity of said aperture of said conductive filmand in contact with said conductive film; wherein said photodiodedetects near-field light that is generated at an interface between saidconductive film and said semiconductor layer by said excited surfaceplasmon.
 2. The photodiode according to claim 1, wherein said conductivefilm is a metal film through which said incident light does not pass atlocations other than said aperture.
 3. The photodiode according to claim1, wherein a region in which a Schottky barrier formed by saidconductive film and said semiconductor layer appears substantiallymatches a region of generation of said near-field light. 4-25.(canceled)
 26. The photodiode according to claim 2, wherein a region inwhich a Schottky barrier formed by said conductive film and saidsemiconductor layer appears substantially matches a region of generationof said near-field light.
 27. The photodiode according to claim 1,wherein said periodic structure is composed of surface irregularitieshaving a period in a direction of increasing distance from saidaperture.
 28. The photodiode according to claim 2, wherein said periodicstructure is composed of surface irregularities having a period in adirection of increasing distance from said aperture.
 29. A photodiodecomprising: a conductive film having a first surface and a secondsurface and including: an aperture having a diameter smaller thanwavelength of incident light that is formed from said first surfaceside; and a periodic structure composed of surface irregularities havinga period in a direction of increasing distance from said aperture; afirst semiconductor layer of one conductive type provided in a vicinityof said aperture of said conductive film and in contact with the secondsurface of said conductive film; and a second semiconductor layer ofsaid one conductive type in which concentration of impurities is higherthan in said first semiconductor layer, and which contacts a surface ofsaid first semiconductor layer that is opposite to another surface incontact with the second surface of said conductive film.
 30. Thephotodiode according to claim 29, wherein said conductive film iscomposed of a metal film, and said surface irregularities are formed insaid first surface.
 31. The photodiode according to claim 29, whereinsaid periodic structure is composed of concentric grooves that take saidaperture as center.
 32. The photodiode according to claim 29, furthercomprising: a first electrode electrically connected to said firstsemiconductor layer and a second electrode electrically connected tosaid conductive film for applying a reverse bias voltage for forming aSchottky barrier in a vicinity of a junction with said conductive filmof said second semiconductor layer; wherein a thickness of said secondsemiconductor layer interposed between said first semiconductor layerand said conductive film is equal to or less than a length of bleedingof near-field light that appears on said first-surface side at alocation of said aperture when light is irradiated onto said conductivefilm from said second surface.
 33. The photodiode according to claim 30,further comprising: a first electrode electrically connected to saidfirst semiconductor layer and a second electrode electrically connectedto said conductive film for applying a reverse bias voltage for forminga Schottky barrier in a vicinity of a junction with said conductive filmof said second semiconductor layer; wherein a thickness of said secondsemiconductor layer interposed between said first semiconductor layerand said conductive film is equal to or less than a length of bleedingof near-field light that appears on said first-surface side at alocation of said aperture when light is irradiated onto said conductivefilm from said second surface.
 34. The photodiode according to claim 31,further comprising: a first electrode electrically connected to saidfirst semiconductor layer and a second electrode electrically connectedto said conductive film for applying a reverse bias voltage for forminga Schottky barrier in a vicinity of a junction with said conductive filmof said second semiconductor layer; wherein a thickness of said secondsemiconductor layer interposed between said first semiconductor layerand said conductive film is equal to or less than a length of bleedingof near-field light that appears on said first-surface side at alocation of said aperture when light is irradiated onto said conductivefilm from said second surface.
 35. The photodiode according to claim 29,wherein said aperture has a bottom surface portion that is a part ofsaid conductive film.
 36. The photodiode according to claim 30, whereinsaid aperture has a bottom surface portion that is a part of saidconductive film.
 37. The photodiode according to claim 31, wherein saidaperture has a bottom surface portion that is a part of said conductivefilm.
 38. The photodiode according to claim 29, wherein a scatteringmember composed of a conductive material for scattering light isarranged in said aperture.
 39. The photodiode according to claim 30,wherein a scattering member composed of a conductive material forscattering light is arranged in said aperture.
 40. The photodiodeaccording to claim 31, wherein a scattering member composed of aconductive material for scattering light is arranged in said aperture.41. The photodiode according to claim 35, comprising a scattering membercomposed of conductive material for scattering light, said scatteringmember being embedded in said second semiconductor layer side from aninterface between said bottom surface portion and said secondsemiconductor layer corresponding to the position of said aperture. 42.The photodiode according to claim 36, comprising a scattering membercomposed of conductive material for scattering light, said scatteringmember being embedded in said second semiconductor layer side from aninterface between said bottom surface portion and said secondsemiconductor layer corresponding to the position of said aperture. 43.The photodiode according to claim 37, comprising a scattering membercomposed of conductive material for scattering light, said scatteringmember being embedded in said second semiconductor layer side from aninterface between said bottom surface portion and said secondsemiconductor layer corresponding to the position of said aperture. 44.The photodiode according to claim 29, wherein said aperture penetratessaid conductive film and reaches said second semiconductor layer, and ofsaid conductive film, a periphery around said aperture contacts saidsecond semiconductor layer.
 45. The photodiode according to claim 30,wherein said aperture penetrates said conductive film and reaches saidsecond semiconductor layer, and of said conductive film, a peripheryaround said aperture contacts said second semiconductor layer.
 46. Thephotodiode according to claim 31, wherein said aperture penetrates saidconductive film and reaches said second semiconductor layer, and of saidconductive film, a periphery around said aperture contacts said secondsemiconductor layer.
 47. The photodiode according to claim 44, wherein ascattering member composed of a conductive material for scattering lightis embedded in a surface of said second semiconductor layercorresponding to the position of said aperture.
 48. The photodiodeaccording to claim 45, wherein a scattering member composed of aconductive material for scattering light is embedded in a surface ofsaid second semiconductor layer corresponding to the position of saidaperture.
 49. The photodiode according to claim 46, wherein a scatteringmember composed of a conductive material for scattering light isembedded in a surface of said second semiconductor layer correspondingto the position of said aperture.
 50. The photodiode according to claim29, wherein a transparent film having an index of refractionsubstantially equal to that of said second semiconductor layer isprovided on said first surface of said conductive film.
 51. Thephotodiode according to claim 30, wherein a transparent film having anindex of refraction substantially equal to that of said secondsemiconductor layer is provided on said first surface of said conductivefilm.
 52. The photodiode according to claim 31, wherein a transparentfilm having an index of refraction substantially equal to that of saidsecond semiconductor layer is provided on said first surface of saidconductive film.
 53. The photodiode according to claim 50, furthercomprising an antireflection film for incident light provided on saidtransparent film.
 54. The photodiode according to claim 51, furthercomprising an antireflection film for incident light provided on saidtransparent film.
 55. The photodiode according to claim 52, furthercomprising an antireflection film for incident light provided on saidtransparent film.
 56. The photodiode according to claim 29, wherein saidconductive film is a metal film and the diameter of said aperture is atleast 1/10 but no greater than ½ the wavelength of said incident light.57. The photodiode according to claim 30, wherein said conductive filmis a metal film and the diameter of said aperture is at least 1/10 butno greater than ½ the wavelength of said incident light.
 58. Thephotodiode according to claim 31, wherein said conductive film is ametal film and the diameter of said aperture is at least 1/10 but nogreater than ½ the wavelength of said incident light.
 59. The photodiodeaccording to claim 56, wherein the period of said periodic structure isequal to or less than the wavelength of said incident light.
 60. Thephotodiode according to claim 57, wherein the period of said periodicstructure is equal to or less than the wavelength of said incidentlight.
 61. The photodiode according to claim 58, wherein the period ofsaid periodic structure is equal to or less than the wavelength of saidincident light.
 62. The photodiode according to claim 56, wherein theperiod of said periodic structure is set to a resonant wavelength of thesurface plasmon excited on said conductive film by said incident light.63. The photodiode according to claim 57, wherein the period of saidperiodic structure is set to a resonant wavelength of the surfaceplasmon excited on said conductive film by said incident light.
 64. Thephotodiode according to claim 58, wherein the period of said periodicstructure is set to a resonant wavelength of the surface plasmon excitedon said conductive film by said incident light.
 65. The photodiodeaccording to claim 56, wherein said metal film has a thickness nogreater than 1000 nm but at least 100 nm at concave portions of saidperiodic structure, and a depth of said surface irregularities is atleast 20 nm but no greater than 200 nm.
 66. The photodiode according toclaim 57, wherein said metal film has a thickness no greater than 1000nm but at least 100 nm at concave portions of said periodic structure,and a depth of said surface irregularities is at least 20 nm but nogreater than 200 nm.
 67. The photodiode according to claim 58, whereinsaid metal film has a thickness no greater than 1000 nm but at least 100nm at concave portions of said periodic structure, and a depth of saidsurface irregularities is at least 20 nm but no greater than 200 nm. 68.The photodiode according to claim 32, wherein a thickness of said secondsemiconductor layer interposed between said first semiconductor layerand said conductive film is at least 50 nm but no greater than 100 nm.69. The photodiode according to claim 33, wherein a thickness of saidsecond semiconductor layer interposed between said first semiconductorlayer and said conductive film is at least 50 nm but no greater than 100nm.
 70. The photodiode according to claim 34, wherein a thickness ofsaid second semiconductor layer interposed between said firstsemiconductor layer and said conductive film is at least 50 nm but nogreater than 100 nm.
 71. A method for fabricating a photodiode which hasa conductive film having an aperture and periodic surface irregularitiesthat takes said aperture as center, and a semiconductor layer joined tosaid conductive film at a position of a bottom of said aperture, themethod comprising the steps of: defining and forming said semiconductorlayer such that a region for carrying out photoelectric conversion islimited to a position corresponding to the bottom of said aperture;forming said conductive film; and forming said aperture and said surfaceirregularities in said conductive film such that said aperture and saidsurface irregularities are matched to said region.
 72. An optical modulecomprising: a photodiode according to claim 1 for detecting signal lightemitted from an optical fiber to supply it as an electrical signal; anda preamplifier for amplifying the electrical signal.
 73. An opticalmodule comprising: a photodiode according to claim 29 for detectingsignal light emitted from an optical fiber to supply it as an electricalsignal; and a preamplifier for amplifying the electrical signal.
 74. Theoptical module according to claim 72, comprising: a case; and an opticalcoupler for optically coupling said optical fiber and said photodiode;wherein said photodiode and said preamplifier are accommodated in saidcase.
 75. The optical module according to claim 73, comprising: a case;and an optical coupler for optically coupling said optical fiber andsaid photodiode; wherein said photodiode and said preamplifier areaccommodated in said case.
 76. An optical interconnection modulecomprising: a photodiode according to claim 1 for receiving incidence oflight emitted from a first optical fiber to generate a first signalcurrent; a light source for generating a signal light that is irradiatedinto a second optical fiber; and a mounting board on which saidphotodiode and said light source are arranged; wherein said first signalcurrent is supplied to an LSI, and said light source generates thesignal light in accordance with the second signal current from said LSI.77. An optical interconnection module comprising: a photodiode accordingto claim 29 for receiving incidence of light emitted from a firstoptical fiber to generate a first signal current; a light source forgenerating a signal light that is irradiated into a second opticalfiber; and a mounting board on which said photodiode and said lightsource are arranged; wherein said first signal current is supplied to anLSI, and said light source generates the signal light in accordance withthe second signal current from said LSI.
 78. The optical interconnectionmodule according to claim 76, further comprising: a first opticalcoupler for optically coupling said first optical fiber and saidphotodiode; and a second optical coupler for optically coupling saidlight source and said second optical fiber.
 79. The opticalinterconnection module according to claim 77, further comprising: afirst optical coupler for optically coupling said first optical fiberand said photodiode; and a second optical coupler for optically couplingsaid light source and said second optical fiber.